1. Field of the Invention
This invention relates to a thick-film resistor containing no lead and a ceramic circuit board.
2. Description of the Prior Art
In forming a resistor on a surface of a ceramic substrate by a thick-film method, a thick-film resistor pattern is conventionally printed using a thick-film resistor paste. The thick-film resistor pattern is then fired to be formed into a thick-film resistor. At present, a mixture of ruthenium oxide and glass is generally used to form a thick-film resistor in order to adjust the firing temperature and resistance value. However, the glass used for the formation of the thick-film resistor contains lead (Pb) for the following reasons:
(1) Electrical resistance of the thick-film resistor is obtained by resistance due to contact of fine powder of an electrically conductive material (ruthenium oxide) and resistance due to a thin film of glass between the conductive materials. However, a quantity of conductive material is reduced when a thick-film resistor to be fabricated has a high resistance at or above 100 kxcexa9/square. As a result, the resistance due to a thin film of glass between the conductive materials is dominant and accordingly, the resistance value tends to be changed even by a slight variation in a firing step. As a countermeasure, a ruthenium composite oxide such as Pb2RU2O6, Bi2Ru2O7, etc., each of which has a higher resistivity than RuO2, is used as the conductive material such that a blending ratio of the conductive material is increased, whereupon a rate of electrical conduction by the contact of the conductive materials is increased.
However, the ruthenium composite oxide partially decomposes in the firing step, thereby rendering the characteristic of the thick-film resistor unstable. For example, Bi2Ru2O7 decomposes as follows:
Bi2Ru2O7xe2x86x922RuO2+Bi2O3
The thick-film resistor contains both RuO2 and Bi2Ru2O7 as the result of the decomposition. Glass used for the formation of the thick-film resistor needs to contain PbO in order that the aforesaid decomposition may be prevented.
(2) When the glass of the thick-film resistor contains PbO, characteristics of the glass such as a melting point, thermal expansion coefficient, etc. can readily be adjusted and accordingly, the characteristics of the thick-film resistor can readily be adjusted. However, the use of lead is undesirable from the point of view of environmental protection. A thick-film resistor using no lead needs to be developed early.
A compressive force needs to be applied from the ceramic substrate to the thick-film resistor to prevent progress of microcrack in order that the stability of thick-film resistor may be ensured for a long period of time. For this purpose, the thermal expansion coefficient of the thick-film resistor needs to be rendered smaller than that of the ceramic substrate. The ruthenium composite oxide has a thermal expansion coefficient of 8.0 to 10.0xc3x9710xe2x88x926/xc2x0 C., which value is rather larger than the thermal expansion coefficient, 4 to 6xc3x9710xe2x88x926/xc2x0 C., of the ceramic substrate. Accordingly, RuO2, (5 to 6xc3x9710xe2x88x926/xc2x0 C.) is desirable as the conductive material for the ceramic substrate. However, since RuO2 has a lower resistivity than the ruthenium composite oxide as described above, the blending ratio of RuO2, needs to be reduced and that of glass needs to be increased when a resistor having the resistance at or above 100 kxcexa9/square is fabricated from an RuO2 thick-film resistor. As a result, the resistance value tends to be changed even by a slight variation in the firing step.
Furthermore, a glass paste is conventionally printed and fired on the surface of a thick-film resistor or thick-film conductor fabricated on the ceramic substrate so that a film of overcoat glass is fabricated. The surface of the thick-film resistor or thick-film conductor is covered with the overcoat glass for insulation of the thick-film resistor or conductor, whereby the electrical characteristic of the resistor or conductor is stabilized.
Although the conventional overcoat glass contains PbO for adjustment of the characteristics such as a firing temperature, thermal expansion coefficient, etc., the use of lead is undesirable from the point of view of environmental protection. A thick-film resistor using no lead needs to be developed early. In view of this point, the prior art has proposed an unleaded overcoat glass containing no Pb component. However, since the proposed unleaded overcoat glass has a large thermal expansion coefficient, the thermal expansion coefficient of the overcoat glass becomes larger than that of the ceramic substrate when the proposed overcoat glass is used for a ceramic substrate having a low thermal expansion coefficient, whereupon the ceramic substrate applies a tensile force to the overcoat glass.
One of important purposes of the overcoat glass is to limit progress of microcrack caused in the thick-film resistor during laser trimming to thereby reduce the variation of the resistance value with age. The overcoat glass needs to apply a compressive force to the thick-film resistor to fully accomplish the purpose. As described above, however, the ceramic substrate applies the tensile force to the overcoat glass. The tensile force reduces the compressive force applied to the thick-film resistor, whereupon the effect of limiting the progress of microcrack is reduced after the laser trimming and the variations in the resistance value with age are increased.
Therefore, a primary object of the present invention is to provide a thick-film resistor which is unleaded or contains no lead, which is hard to be influenced by the variations in the firing step, which has a stable resistance value, which can be fabricated efficiently, and which can improve the productivity and quality.
Another object of the invention is to provide a ceramic circuit board which uses an overcoat glass which is unleaded and in which characteristics of the overcoat glass such as the firing temperature, thermal expansion coefficient, etc. can properly be adjusted without use of lead component.
To achieve the primary object, the present invention provides a thick-film resistor comprising RuO2 and an SiO2xe2x80x94B2O3xe2x80x94K2O glass having a composition of 60 wt %xe2x89xa6SiO2xe2x89xa685 wt %, 15 wt %xe2x89xa6B2O3xe2x89xa640 wt %, 0.1 wt %xe2x89xa6K2Oxe2x89xa610 wt %, and impurity xe2x89xa63 wt %.
The thick-film resistor is fired at or below 900xc2x0 C. in most cases and more specifically at about 850xc2x0 C. The reason for this is that a metal with a low melting point, for example, Ag or Au, is used as a surface conductor of the ceramic substrate. Another reason is for prevention of evaporation of RuO2. In order that the thick-film resistor may be fired at 850xc2x0 C., it is desired that glass contained in it have a transition point at or below 650xc2x0 C. In the present invention, the SiO2xe2x80x94B2O3xe2x80x94K2O glass contained in the thick-film resistor has the above-described composition such that a transition point thereof is at or below 650xc2x0 C. As a result, the thick-film resistor can be fired at 850xc2x0 C. In this case, K2O contained in the glass serves to lower the glass transition point. Accordingly, when an amount of K2O is smaller than 0.1 wt %, the glass transition point becomes higher than 650xc2x0 C., whereupon it is difficult to fire the thick-film resistor at 850xc2x0 C. Na2O or Li2O can lower the glass transition point, instead of K2O. However, when Na2O or Li2O is used, a temperature coefficient of resistance (TCR) changes to a large extent to thereby take a negative value. This deteriorates the temperature characteristic of the thick-film resistor. Since K2O is used in the present invention, the glass transition point can be lowered without deterioration of the temperature characteristic of the thick-film resistor. The thermal expansion coefficient (TEC) of the glass is increased when a quantity of K2O contained in the glass is excessively increased. Accordingly, it is not preferable to unnecessarily increase a quantity of K2O to be blended into the glass. The thermal expansion coefficient of the glass is at or below 6.0xc3x9710xe2x88x926/xc2x0 C. when the quantity of K2O is at or below 10 wt % as in the present invention. Consequently, a thermal expansion coefficient of the thick-film resistor blended with RuO2 having a thermal expansion coefficient of 5 to 6xc3x9710xe2x88x926/xc2x0 C. is at or below 6.0xc3x9710xe2x88x926/xc2x0 C. Accordingly, when the thick-film resistor is fabricated on a ceramic substrate having a low thermal expansion coefficient (4 to 6xc3x9710xe2x88x926/xc2x0 C.), a compressive force or a slight tensile force is applied to the thick-film resistor. Thus, the thick-film resistor is not subjected to a large tensile force. As a result, the change in the resistance value after the laser trimming is small and accordingly, a stable thick-film resistor can be obtained.
In a first preferred form, RuO2 has a specific surface area ranging between 30 and 80 m2/g. An electric charge tends to be concentrated more as an specific surface area of RuO2 is rendered small, whereupon an electrostatic discharge (ESD) characteristic tends to be reduced. According to results of experiments carried out by the inventors, a preferable ESD characteristic can be ensured when the specific surface area of RuO2 is at or above 30 m2/g. However, when the specific surface area exceeds 80 m2/g, an oxidation catalytic action of RuO2 is intensified such that there is a possibility of spontaneous fire of organic substance. Accordingly, the specific surface area of RuO2 is preferably at or below 80 m2/g.
In a second preferred form, K2O of 0.8 to 4 wt % relative to RuO2 of 100 wt % adheres to a surface of RuO2. K2O is caused to adhere to the surface of RuO2 in a fabrication step of RuO2. K2O on the surface of RuO2 improves wettability between the SiO2xe2x80x94B2O3xe2x80x94K2O glass and RuO2 and accordingly stabilizes the conduction through glass, so that changes in the resistance value due to the changes of the firing temperature or the firing temperature dependency can be reduced. K2O on the surface of RuO2 further prevents the concentration of electric charge, improving the ESD characteristic. The above-mentioned effects are small when K2O on the surface of RuO2 is below 0.8 wt %. Further, when K2O on the surface of RuO2 exceeds 4 wt %, the temperature coefficient of resistance changes to a large extent to thereby take a negative value. Accordingly, an amount of K2O adherent to the surface of RuO2 preferably ranges between 0.8 and 4 wt %.
In a third preferred form, the thick-film resistor further comprises additive glass containing a transition metal oxide and B2O3. Borate containing the transition metal in the additive glass has conductivity by electronic conduction and accordingly a semiconductive property. Accordingly, the additive glass prevents local concentration of electric charge when a surge voltage is applied thereto, thereby preventing breaking of the glass of the thick-film resistor. This characteristic increases the effect of adding the additive glass when a sheet resistance value is large. However, the temperature of coefficient of resistance takes a negative value since the additive glass has the semiconductive property. Thus, in order that both the temperature coefficient of resistance and the ESD characteristic may be satisfied, a quantity of the additive glass preferably ranges between 3 and 15 wt %, for example, when the thick-film resistor has the resistance of 100 kxcexa9/square.
In a fourth preferred form, the thick-film resistor further comprises 5 wt % or less of a transition metal oxide. When a quantity of the transition metal oxide added to the thick-film resistor is at or below 5 wt %, the quantity is adjusted so that the temperature coefficient of resistance of the thick-film resistor can optionally be adjusted. When the quantity of the transition metal oxide exceeds 5 wt %, there is a possibility that the resistance value is not stable. On the other hand, the thick-film resistor self-heats when electric current flows therethrough, so that the temperature thereof is increased. The thick-film resistor varies its resistance value with changes in the temperature thereof. Accordingly, an amount of change In the resistance value of the thick-film resistor is rendered larger as an amount of increase in the temperature due to generation of heat becomes large, whereupon stable electric characteristics cannot be obtained. Further, when the temperature of the thick-film resistor is increased over the critical heat resistance, crack occurs in the thick-film resistor. In view of both electrical characteristics and heat resistance, it is desired that the heat radiation characteristic be increased so that the increase in the temperature due to heat generation is rendered as small as possible. However, glass used in the thick-film resistor has a small heat conductivity for improvement of the electrical characteristios such as the temperature coefficient of resistance (TCR), ESD, etc. As a result, the heat conductivity and heat radiation of the thick-film resistor are deteriorated. Accordingly, in order that the increase in the temperature due to heat generation may be limited within an allowable range, the maximum power supplied to the thick-film resistor needs to be reduced so that an amount of heat generated by the thick-film resistor is reduced and accordingly, the power resistance (heat resistance) is lowered. Particularly when the thick-film resistor is fabricated on a glass ceramic substrate having a small heat conductivity, the heat radiation of the thick-film resistor is further reduced and the power resistance is further lowered.
In a fifth preferred form, the SiO2xe2x80x94B2O3xe2x80x94K2O glass contains 1 to 20 wt % of ZrO2 particle in view of the above-described problem. The ZrO2 particle has a higher heat conductivity than glass and the ZrO2 particle and the SiO2xe2x80x94B2O3xe2x80x94K2O glass do not act on each other at the firing temperature of the thick-film resistor (at or below 900xc2x0 C.). Consequently, when a suitable quantity of ZrO2 is added to the SiO2xe2x80x94B2O3xe2x80x94K2O glass, the thermal conductivity of the thick-film resistor can be increased without deterioration of the electrical characteristics of the thick-film resistors. This can increase heat radiation and improve the power resistance. The results of experiments which will be described later show that a proper range of a quantity of the ZrO2 particle to be added is between 1 and 20 wt %.
However, when the ZrO2 particle is non-uniformly distributed in the thick-film resistor, local heat generation occurs in a portion of the thick-film resistor wherein a quantity of the ZrO2 particle is small. A local increase in the temperature lowers the power resistance and accordingly reduces the effect of addition of the ZrO2 particle. The results of experiments carried out by the inventors show that when the ZrO2 particle has a large particle diameter, it is difficult to uniformly distribute the ZrO2 particle over the thick-film resistor, whereupon the effect of addition of the ZrO2 particle is reduced.
In a sixth preferred form, the ZrO2 particle has a mean particle diameter (D50) which is at or below 3 xcexcm. This particle diameter can substantially uniformly distribute the Zro2 particle over the thick-film resistor.
To achieve the second object, the invention provides a ceramic circuit board comprising a thick-film resistor and/or a thick-film conductor each formed on a ceramic substrate, the thick-film resistor containing RuO2 having a surface to which K2O of 0.8 to 4 wt % relative to RuO2 of 100 wt % adheres and an SiO2xe2x80x94B2O3xe2x80x94K2O glass having a composition of 60 wt %xe2x89xa6SiO2xe2x89xa685 wt %, 15 wt %xe2x89xa6B2O3xe2x89xa640 wt %, and 1 wt %xe2x89xa6K2Oxe2x89xa610 wt %, and impurity xe2x89xa63 wt %, and an overcoat glass covering a surface or surfaces of the thick-film resistor and/or the thick-film conductor and comprising an SiO2xe2x80x94B2O3xe2x80x94K2O glass and impurity, the SiO2xe2x80x94B2O3xe2x80x94K2O glass having a composition of 60 wt %xe2x89xa6SiO2xe2x89xa685 wt %, 15 wt %xe2x89xa6B2O3xe2x89xa630 wt %, and 1 wt %xe2x89xa6K2Oxe2x89xa610 wt %, and a film of the overcoat glass is formed on the thick-film resistor.
The overcoat glass may be fired with the thick-film resistor. However, since the overcoat glass is sometimes printed and fired after the firing of the thick-film resistor, the firing temperature of the overcoat glass is preferably set to be equal to or lower than the firing temperature of the thick-film resistor (for, example, about 850xc2x0 C.). The transition point of the overcoat glass is preferably at or below 550xc2x0 C. in order that the overcoat glass may be fired at a temperature lower than 850xc2x0 C. (for example, in a range between 600 and 700xc2x0 C.).
The glass transition point of the SiO2xe2x80x94B2O3xe2x80x94K2O glass composed as described above is at or below 550xc2x0 C. Thus, the overcoat glass can be fired at a temperature lower than the firing temperatures of the thick-film resistor and the thick-film conductor. In this case, K2O contained in the overcoat glass serves to lower the glass transition point. Accordingly, when K2O is at or below 1 wt %, the glass transition point exceeds 550xc2x0 C. such that it is difficult to reduce the firing temperature of the overcoat glass.
The thermal expansion coefficient (TEC) of the overcoat glass is increased when a quantity of K2O contained in the overcoat glass is excessively increased. Accordingly, it is not preferable to unnecessarily increase the quantity of K2O to be blended into the glass. The thermal expansion coefficient of the overcoat glass is at or below 6.0xc3x9710xe2x88x926/xc2x0 C. when the quantity of K2O is at or below 10 wt % as in the present invention. Accordingly, in a case where this overcoat glass is used on a ceramic substrate having a low thermal expansion coefficient (4 to 6xc3x9710xe2x88x926/xc2x0 C.), the difference between the thermal expansion coefficients of the overcoat glass and the ceramic substrate is small than in the prior art both when the coefficient of the overcoat glass is smaller and larger than that of the ceramic substrate. Accordingly, even when the overcoat glass in accordance with the invention is used for the ceramic substrate having a low thermal expansion coefficient, a compressive force or a slight tensile force is applied from the ceramic substrate to the overcoat glass. Thus, the overcoat glass is not subjected to a large tensile force. As a result, the overcoat glass sufficiently serves to limit the progress of microcrack in the thick-film resistor after the laser trimming. Thus, the change in the resistance value after the laser trimming is small and accordingly, the stability in the resistance of the thick-film resistor can be improved.
The present invention may be applied irrespective of types of the thick-film resistors and conductors. A larger effect can be obtained when the thick-film resistor contains RuO2 and an SiO2xe2x80x94B2O3xe2x80x94K2O glass having a composition of 60 wt %xe2x89xa6SiO2xe2x89xa685 wt %, 15 wt %xe2x89xa6B2O3xe2x89xa640 wt %, 0.1 wt %xe2x89xa6K2Oxe2x89xa610 wt %, and impurity xe2x89xa63 wt %.
When the used thick-film resistor contains the SiO2xe2x80x94B2O3xe2x80x94K2O glass having substantially the same composition as the overcoat glass in accordance with the present invention, the adhesion between the overcoat glass and the thick-film resistor can be Improved, whereupon the protecting effect of the overcoat glass can be improved. Moreover, since the SiO2xe2x80x94B2O3xe2x80x94K2O glass is used as the glass contained in the thick-film resistor, the firing temperature and thermal expansion coefficient of the thick-film resistor can be decreased. Consequently, a high quality unleaded thick-film resistor can be fabricated.
Furthermore, the ceramic substrate is preferably fabricated from a glass ceramic fired at 800 to 1000xc2x0 C. Consequently, since the ceramic substrate, thick-film resistor and overcoat glass can simultaneously be fired in a single firing step, the fabricating efficiency can be improved and the strength of junction of the ceramic substrate and thick-film resistor can be improved.